The present invention relates to wire wound inductors and, in particular, to wire wound inductors utilizing an extruded core material along with simplified terminal attachment and wire windings in order to reduce inductor manufacturing costs.
Inductors form an integral component of radio frequency (RF) circuits. As a group, inductors form about 1/3 of the basic building blocks for circuit design.
The basic form of inductors is a wire coil. The coil can be free-standing (air-core) or wrapped around a core. Other versions of inductors (such as multi-layer or printed design) are known; however, superior performance is achieved from a coil. With the advent of surface-mount technologies for high-speed manufacturing of printed circuit boards, the size of inductors has greatly decreased. Surface-mounted, wire-wound inductors are currently available in industry standard 0805 and 0603 size packages. These inductors consist of a molded core material (either a thermoset plastic or a ceramic) with wire windings and plated terminals.
The electrical measurement unit for inductance is Henries. To the first order approximation, the inductance value of a wire coil is L=(4 .pi.N.sup.2 A/W).times.10.sup.-9 Henries, where N is the number of turns in the coil, A is the cross sectional area of the coil, and W is the length of the coil. All three variables (N, A, and W) are independent such that they can be independently varied to obtain a desired inductance value L.
Inductors are currently manufactured one at a time with the wire ends of the windings being bonded while the inductor is in the winding fixture. This method is time consuming, resulting in increased manufacturing costs and can result in less than desirable tolerance deviations. In addition, conventional inductors utilize core materials that cannot be extruded in large quantities and thus cannot take advantage of a continuous process. Moreover, the conventional core materials are difficult to machine, and as a result, the cross sectional area of the coil can be difficult to determine accurately. Still further, terminals in the conventional inductors are coplanar (i.e., on the same side of the inductor), and the wire windings begin and terminate on the same side (typically the bottom) of the device. As a result, only integer multiples of windings are possible (N in the above equation for Henries). In turn, this limits the number of inductance values (L in the above equation) obtainable for a given core size. Still further, an adhesive coating (particularly a UV or heat cured plastic) is added to wire wound surface-mountable inductors in order to secure the wire windings and to provide a smooth, uniform surface for automated placement devices. Since the coating material can run over the edges of the device, an external mold may be required to provide a uniform surface.